Integrated circuits (ICs) with high voltage capabilities have wide industrial applications, including power management systems used in automobiles. These integrated circuits include high voltage transistors that operate in a high voltage range (e.g., 80V to 120V) and low voltage transistors that operate in a much lower voltage range (e.g., 1V to 5V). To protect the low voltage transistors from the high voltage operations, an IC may adopt one or more electrical isolation schemes. For instance, one isolation scheme involves junction isolation comprising forming a buried layer (e.g., an N-type buried layer) within a bulk substrate and a sinker diffusion extending from the buried layer to the top surface of a bulk substrate (e.g., a P-type substrate) to junction isolate the high voltage transistors from the low voltage transistors. The N-type buried layer is typically biased by connecting it to a high voltage corresponding to the operating range of the high voltage transistors, whereas the P-type substrate is biased by connecting it to the lowest potential on the IC typically being ground. In some cases, the difference between the high bias voltage and the ground voltage may exceed a breakdown voltage threshold of a PN junction between the buried layer and the substrate, thereby causing leakage and impacting the performance and reliability of the IC.
Junction isolation may be combined with trench isolation. In this case dielectric filled trenches extend down to the buried layer and are surrounded by the sinker to form isolation trenches.